Ductwork is often used for conveying conditioned air (e.g., heated, cooled, filtered, etc.) discharged from a fan and distributing the air to a room or other areas within a building. Ducts are typically formed of rigid metal, such as steel, aluminum, or stainless steel. In many installations, ducts are hidden above suspended ceilings for convenience and aesthetics. But in warehouses, manufacturing plants and many other buildings, the ducts are suspended from the roof of the building and are thus exposed. In those warehouse or manufacturing environments where prevention of air-borne contamination of the inventory is critical, metal ducts can create problems.
For instance, temperature variations in the building or temperature differentials between the ducts and the air being conveyed can create condensation on both the interior and exterior of the ducts. The presence of condensed moisture on the interior of the duct may form mold or bacteria that the duct then passes onto the room or other areas being supplied with the conditioned air. In the case of exposed ducts, condensation on the exterior of the duct can drip onto the inventory or personnel below. The consequences of the dripping can range anywhere from a minor irritation to a dangerously slippery floor or complete destruction of products underneath the duct (particularly in food-processing facilities).
Further, metal ducts with localized discharge registers have been known to create uncomfortable drafts and unbalanced localized heating or cooling within the building. In many food-processing facilities where the target temperature is 42 degrees Fahrenheit, a cold draft can be especially uncomfortable and perhaps unhealthy.
Many of the above problems associated with metal ducts are overcome by the use of flexible fabric ducts, such as a Frommelt DUCTSOX. Such ducts typically have a flexible fabric wall (often porous) that inflates to a generally cylindrical shape by the pressure of the air being conveyed by the duct. Fabric ducts seem to inhibit the formation of condensation on its exterior wall, possibly due to the fabric having a lower thermal conductivity than that of metal ducts. In addition, the fabric's porosity and/or additional holes distributed along the length of the fabric duct broadly and evenly disperse the air into the room being conditioned or ventilated. The even distribution of airflow also effectively ventilates the walls of the duct itself, thereby further inhibiting the formation of mold and bacteria.
In many cases, however, once the room's conditioning demand has been met, the air supply fan is turned off or down until needed again. When the fan is off, the resulting loss of air pressure in the duct deflates the fabric tube, causing it to sag. Depending on the application and material of the fabric, in some cases, the sagging creates a poor appearance or may interfere with whatever might be directly beneath the duct. Moreover, when the duct is re-inflated, the duct can produce a loud popping sound as the duct's fabric becomes taut.
To eliminate or reduce the sagging and popping noise, some inflatable ducts include structure that helps hold a deflated duct in a generally expanded shape. Examples of ducts supported in such a manner are disclosed in U.S. Pat. Nos. 6,280,320 and 3,357,088. A significant drawback of the patented systems is the amount of supporting hardware necessary to keep the duct expanded. For the air duct of the '320 patent, various embodiments include two parallel support channels (FIGS. 1-9), an assembly comprising numerous components (items 80, 94, 74, 82, and 84 of FIG. 12), or large cumbersome hoops (FIGS. 13 and 14).
For the air duct of the '088 patent, the support structure is similar to a triangular coat hanger comprising three structural bars (items 19, 20 and 21). Bar (21) of the '088 patent extends through the interior of the duct, which can disrupt the airflow. The '088 device also includes grommets 23 through which the structural bars extend. If the holes in the grommets are too big, the grommets may slide around the structural bars, which would allow the duct to sag. If the holes in the grommets are too small, the resulting tight fit between the grommets and the structural bars would make it more difficult to remove the bars for periodic laundering of the fabric duct.
Consequently, a need exists for a simple, lightweight structure that can support a deflated duct in a generally expanded shape.